Most conventional concave lenses usually have thin intermediate parts thereof. Thus, a quantity of molten resin material can barely be fed into the intermediate portion of the mold until a full quantity of resin can flow into the mold. The results are that a concave lens formed within the mold has a weld line in the radial directions partly due to the orientation of the flow and partly due to the difference in curing rate of the molten resin that has been fed. Because of the above mentioned facts, lenses formed by means of the instruments generally in wide use cannot be practically employed in the form of concave lenses, or more precisely, concave lenses cannot be produced through process of injection molding.